For a packaging body formed by filling a container with contents such as food and heat-sealing a cover to the container, high sealing performance is desired in terms of keeping quality of the contents, which requires sealing with high heat-sealing strength. On the other hand, in terms of usability in using the contents, the packaging body should preferably be opened with small force, in other words, easy-open performance is desired. Various methods have been suggested for satisfying the sealing performance and easy-open performance as contradictory performances.
Especially, there have been suggested a method for adjusting adhesive strength of a heat-sealed interface by controlling heat-sealing condition in sealing a container by heat-sealing a flange extending from an opening of the container in an outer circumferential direction and a cover, and a method for forming a container such that at least an innermost layer of the container is peeled off with a cover when the cover is opened.
In terms of securing rigidity of the container to meet size increase, shape change, and thickness reduction of the container for resource saving, it has been suggested increasing the width of a flange of the container, providing a flange with a skirt having a bent part that is formed by bending an outer circumferential end of the flange; providing a flange with a curl formed by bending an outer circumferential end of the flange to have a substantially C-shaped cross section.
In the container with the uniquely-shaped flange having the skirt or the curl, when the flange of the container and the cover are heat-sealed, it is required to peel off the innermost layer of the container with the cover while leaving a part other than a heat-sealed part of the flange. Thus, it might become even more difficult to realize the easy-open performance while keeping the sealing performance.
In order to solve the problem, a new easy-open container has been suggested (see, for instance, Japanese Patent Publication No. 2724355, left column of page 3 and FIG. 1).
In the disclosed technique, ringed inner and outer notches are respectively provided on the inner and outer circumferential sides of a flange to enclose an opening of the container, and a melt-seal part is formed between the inner and the outer notches so that the flange and the cover adhere to each other. A projected seal part projected outward relative to the outer notch is formed on the melt-seal part on a position corresponding to a tab of the cover. A dent is formed on an inner layer of the container positioned on an outer edge of the projected seal part. In the easy-open container, when the cover is peeled off from the container, the dent formed on the outer edge of the projected seal part serves as a trigger of the peeling, and subsequently, the cover is gradually peeled off along the outer notch.
However in the technique described above, when the contents of the container are food, the contents are contained and then boiled or heated for sterilization after forming the container and the notches. Since positions of the notches are slightly displaced due to the heating, a heat-seal position cannot be placed between the two notches and the heat-sealing is provided on an area out of the notches, which causes problem in sealing performance of the container. If, for instance, adhesive strength between the cover and the container is increased to solve the problem of the sealing performance, then peeling with small force becomes difficult, so that the easy-open performance cannot be realized. As a result, both of the sealing performance and the easy-open performance cannot be achieved at the same time.
In addition, in forming the dent on the outer edge of the projected seal part, a dent is formed on a position other than the predetermined position described above, which causes the peeling to start from the outer circumferential end of the heat-sealed part instead of from the outer notch, so that it is difficult to manufacture the container with consistent quality.